In the field of automobile industry, application of plastics (synthetic resins) to various automotive parts has been eagerly studied with the recent tendencies toward weight reduction and energy saving. Polyolefin resins have generally been used as plastic materials for their cheapness, high strength, weather resistance and chemical resistance.
High-molecular weight and high-density polyethylene has been proposed as a plastic material for a fuel tank, but it was reported that its gas barrier properties are not always sufficient so that gasoline evaporates to pollute the environment.
A multi-layer container with improved gas barrier properties has been proposed in JP-B-61-42625, which is composed of a modified nylon resin layer having laminated on one side thereof a polyolefin resin layer via an adhesive layer. (The term "JP-B" as used herein means an "examined Japanese patent publication".) The modified nylon resin layer comprises nylon and a molten mixture of a modified copolymer having a maleic anhydride content of from 0.05 to 1% by weight prepared by grafting maleic anhydride to an ethylene-.alpha.-olefin copolymer having a degree of crystallinity of from 1 to 35% and a melt index of from 0.01 to 50 g/10 min. The adhesive layer comprises a modified copolymer having a maleic anhydride content of from 0.01 to 1% by weight prepared by grafting maleic anhydride to an ethylene-.alpha.-olefin copolymer having a degree of crystallinity of from 2 to 30% and a melt index of from 0.01 to 50 g/10 min.
The polyolefin resin layer used in the above-mentioned multi-layer container specifically includes "Novatec BR300", a registered trade name of Mitsubishi Chemical Corp., which is high-density polyethylene having a zero shear viscosity of 1.25.times.10.sup.7 poise at 190.degree. C.
However, the multi-layer container using high-density polyethylene having a zero shear viscosity of 1.25.times.10.sup.7 poise at 190.degree. C. as a polyolefin resin layer has a disadvantage of poor Izod impact strength particularly at low temperatures. Moreover, thin-walled parts are apt to be occur in the upper portion of the multi-layer container due to its poor resistance to drawdown, and the polyolefin resin layer is apt to become non-uniform due to poor uniform melt ductility.
Recycling of molding flash or salvaged material has been considered for cost reduction of plastic fuel tanks. However, reuse of the flash or salvaged material of conventional multi-layer plastic fuel tanks involves a problem that the resulting fuel tanks have reduced impact resistance.
In order to improve impact resistance, it has been suggested to knead high-density polyethylene and a part or the whole of salvaged multi-layer plastic fuel tanks (20 to 200 parts by weight per 100 parts by weight of the high-density polyethylene) in an extruder having an L/D ratio of 20 or more and a specially shaped screw at an extruding temperature of 200.degree. to 250.degree. C., as described in JP-A-4-47918. (The term "JP-A" as used herein means an "unexamined published Japanese patent application").
The above-described process, however, requires to control the kneading temperature and to employ a specially shaped screw in a kneading machine. Further, it is difficult to produce a fuel tank retaining superiority in all aspects of barrier properties, impact resistance, weather resistance, and chemical resistance. Furthermore, it is not expected to inhibit reduction in impact resistance due to reuse of molding flash or salvaged material.
The present invention has been accomplished in the light of the above-described circumstances.